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Darwin the botanist

Volume 5 Number 2 - October 2008
David Kohn

French title: Darwin le botaniste

Spanish title: Darwin el botánico

Summary

In his theory of evolution, Charles Darwin proposed that living beings were “all netted together” through a common ancestry. Yet one branch of the tree of life, the plant kingdom, was Darwin’s most enduring focus. He was a major field collector, a consummate and sustained observer of plant life, a rigorous botanical experimentalist, and the high theorist of plant evolution.

Botany played a pivotal part in each phase of Darwin’s life. As an undergraduate, he collected specimens for his botany professor’s herbarium while on a geological expedition in Wales. Voyaging for five years aboard the HMS Beagle, he collected plants along with fossil bones and bird skins. Preparing to write On the Origin of Species, botany became critical to the growth of his evolutionary theory. Ultimately, he turned his home and the surrounding countryside into a botanical field station.

Darwin’s six botanical books would recast large areas of plant science. His studies on the fertilization of orchids, on insectivorous plants, and on the climbing and other plant movements were each a beautifully articulate example of how evolution could solve the traditional mysteries of natural history. Through his work, Darwin laid foundations for modern botany that remain firm to this day.

Darwin’s botanical formation

Darwin was heir to a rich botanical tradition, one that often resonated in the specific botanical topics and viewpoints he was to adopt. His grandfather Erasmus Darwin, for example, was an evolutionist and spread the Linnaean doctrine that plants, just like animals, have sexes. Darwin’s first botanical exposure was in his father’s ample garden at The Mount in Shrewsbury where young Charles played among apple trees bred by Thomas Andrew Knight, President of the Royal Horticultural Society. However, the foundations of Darwin’s formal education in botany were laid at Cambridge by his dynamic professor John Stevens Henslow (1796-1861) who later opened the way for Darwin to participate in the Beagle’s scientific cruise around the world.

The Beagle voyage

The Beagle expedition concentrated on certain regions of South America: coastal Brazil, Patagonia, Tierra del Fuego, the Andean coast as far north as Lima and the oceanic Galápagos archipelago.

Plants came before birds in the birth of Darwinian evolution. Darwin did not become an evolutionist on the Galápagos, but the basis for a profound shift in his understanding of species was established there – and it began with plants. When the Beagle arrived in the Galápagos in September 1835, Darwin immediately observed that the flora appeared unique. Thereupon, he collected “all the plants in flower”. This we know from a pocket field notebook that Darwin kept when he was in the archipelago. Darwin’s Galápagos plant specimens, numbering well over 200, constitute the single most influential natural history collection of live organisms in the entire history of science. Indeed, Darwin’s plants represent the foundational collection for the entire Galápagos flora. Settled back in London in 1837, he sketched his first evolutionary tree and began firmly applying the idea of descent with modification to all of natural history.

In 1856, Darwin put ink to blue foolscap and began to steadily write. His seminal book, On the Origin of Species, was finally published in November 1859.

Preserving priority

Once the Origin was published, botany became the central focus of Darwin’s research for the remaining years of his life. This botanical work was highly original, not only because of the quality of his observations, but because it was the first attempt to apply the principles of evolution to plants.

Darwin’s crucial contribution to botany was his understanding of and ability to demonstrate that the flower is a product of evolution. By the 18th century, botanists were convinced that flowering plants have both male and female parts, and assumed that most plants self fertilise or inbreed. This erroneous and deeply ingrained view of the flower continued until Darwin began publishing on the biological meaning of flowers in the 1860s. The prevalence of cross-pollination was Darwin’s single most important botanical truth.

Indeed Darwin realised there was a problem with plant sex immediately after discovering natural selection in 1838. If flowers perpetually self fertilised then both natural selection and evolution would be invalid, because natural selection requires hereditary variation. If there is no variation, there is simply nothing to select. Evolution stops. If flowers self fertilised for generation after generation, they would become unvarying clones. Furthermore, without variation, where did the thousands of known plant species come from? Evolution would not explain the diverse families of flowering plant genera and species. The world’s flowers were just too big a part of nature for Darwin to forfeit them to the creationist position that species are immutable.

In the case of flowers, he undertook decades of field and garden observations and breeding experiments, all focused on testing and supporting one powerful hypothesis: that of natural selection.

Through his painstaking studies, such as the Primula example below, Darwin discovered that flowering plants have evolved elaborate structures, strategies, and relations with animals and reasoned that it was all to avoid the apparent necessity of perpetual inbreeding. Thus the Darwinian meaning of flowers became a pillar of botany and botany became one of the strongest fields supporting evolution.

Love, Primula style

Spring 1860 was the first flowering season following Origin’s publication. Darwin, bursting with experimental energy, made several crucial botanical discoveries. In May, he observed two different kinds of flowers among common primulas. The style, or shaft of the female part, is either tall, protruding like a pin, or it is short. Darwin’s children gathered armloads of flowers for their father, who noticed something: The two kinds of flower occur in a 50:50 ratio. To explain these two morphs, Darwin would follow the scientific method, but after his very own personal style. At first, the 50:50 ratio reminded him of a normal male-female sex ratio. So he believed the two kinds of flowers were evolving into separate sexes. That is, he thought he had witnessed one step in the evolution of separate male and female flowers. The tall-style flowers must be evolving into pure females, he surmised. Conversely, their short stamens must be losing potency. To test this he crossed the two forms. But to his surprise, the ‘males’ produced abundant seeds. So the experimental method forced him to abandon his first hypothesis. As Darwin once observed, his first explanations of things frequently proved wrong. The originality and breakthrough would come with thinking up the next explanation.

He now realised that the maximum fertility occurs when pollen moves from one form to the other. It was always the self fertilised flowers that had reduced fertility. Thus the two forms, each of which is both male and female, are favoured to maintain a stable population. Darwin had in fact discovered a breeding strategy that gave a clear advantage to cross-pollination and thus provided experimental support for his long-held interpretation of the meaning of flowers. To think of flowers in terms of plant breeding strategies, now that is evolutionary botany. And Darwin’s primulas have been a prime example of this new way of thinking ever since.

Plant sensitivity: green adaptations

Plants don’t have feet and they also don’t have brains. But as far as Darwin was concerned, some plants as good as have eyes, and they do have behaviour, often expressed in the extraordinary ways these beings can move despite being rooted in the soil. Darwin’s passion for working out the botanical adaptations that allow plants to stay in sensitive touch with their environment occupied him for years in physiological experiments that prefigure the biochemical and cellular studies of the early 20th century on enzymatic catalysis and plant hormones. Thus these green adaptations in leaves and stems not only paralleled Darwin’s flowers, they added a new level of sophistication to the botanical bulwarks Darwin had built to support evolution and adaptation by natural selection. For example, insectivorous plants that trap and digest insects with specialised leaves fascinated Darwin. So did vines that climb up and over other plants. He saw plants as sensitive creatures whose growing tips or leaves or seedlings can track the movement of the sun. Indeed, he showed they could respond to the least beam of incident light, the pull of gravity, and the slight touch of a browsing animal.

Much of this work Darwin performed in a string of hothouses that gradually sprang up along the kitchen garden wall beginning in the late 1850s. Eventually, there were five houses heated by a boiler and offering a fair range of conditions for the array of plants that Darwin, assisted in his last years by his son Francis, wished to study.

Time-lapse time machine

Darwin was also keenly aware of the cell theory, and by the end of his life in April 1882 was venturing quite earnestly with Francis into what we now call biochemistry and cell biology. Darwin believed there must be cellular processes underlying, for example, a sundew’s ability to trap insects. He used a James Smith achromatic compound microscope to visually capture what went on at the cellular level when the tentacles of a Drosera rotundiflora curl over a fly that has landed on one of its leaves. In his field of vision he focused on a single cell from the tentacle of a sundew that had earlier been ‘fed’. Over a period of an hour he drew the same cell seven times as he observed progressive changes in what he considered to be the cell’s protoplasm. He called the process aggregation. He also observed that as a tentacle relaxes, the aggregation disappears. Darwin compared this reversible process, which he thought required a ferment (soon afterwards called an enzyme), “with a reflex action in the nervous system of an animal”.

What he actually saw, by modern lights, has not been established and is controversial to this day. Today, mechanisms for communication between plant cells are at the forefront of research. But the point is just how far Darwin pushed his Drosera studies towards experimental biology while pursuing his evolutionary physiological botany.

Power of movement: Are plants alive?

Darwin wrote six botanical books, over 75 articles, and well-articulated and rigorously executed studies. He titled his last plant book The power of movement in plants (1880). Fascination with the ways that plants move entered the early botanical literature in conjunction with a difficult question: Are plants alive? Although they don’t have evident nervous systems, they can respond to irritation in some cases. This was taken as a sign of life. At the bottom is the even more difficult question, ‘What is life?’ For Darwin, the traditional ‘What is life?’ question was transformed into an effort to demonstrate the unity of all life and hence, by implication, the common descent of all branches of the evolutionary tree. This is the unspoken thesis of Power of Movement. Darwin was once again universalising. By showing that plants have a power of movement and given that mobility and the capacity for movement are animal-like characteristics, he is supporting the unity of common descent, which in turn is an underlying assumption or implication (depending on how the argument is phrased) of evolution.

Darwin and the foundation of evolutionary botany

As we have seen, Darwin not only contributed to botany, he actually changed the discipline by his very contributions. Since all of his botanical researches were conducted as applications of the theory of evolution, replete with well-worked examples often treated as evidence for natural selection, he was using botany to defend his theory. But simultaneously he was also providing botanists with a model for how to think about their own observations in evolutionary terms. Those who followed Darwin’s botanical writings would have seen that their discipline, and indeed the plant kingdom, was full of examples of evolution. It was not left for late-19th-century botanists, as it largely was for zoologists, to apply the concepts in the Origin on their own. Darwin directly led the way for his botanical followers. Thus he was the first openly practicing evolutionary botanist in the midst of a community that included some botanists who were - and some who weren’t - swayed by evolution. Darwin was so invested in his plant research that he himself built the bridge directly from the Origin to fundamental problems in botanical science. Thus did Darwin assist at the birth of evolutionary botany.

This dual role as both founder of evolution by natural selection and exponent of how the theory could be applied in botany is quite remarkable. Later generations of Darwinian botanists would complete the transformation of botany into evolutionary science. But Darwin went a long way in providing a model for that transformation.

Résumé

Dans sa théorie de l’évolution, Charles Darwin avançait que les êtres vivants étaient « tous reliés les uns aux autres » à travers un ancêtre commun. Toutefois Darwin porta une attention des plus vives à l’une des branches de l’arbre de la vie, le règne végétal, source intarissable de sa joie. Il étudia les plantes avec cohérence et profondeur au cours d’une longue carrière scientifique. Il s’agissait d’un grand homme de terrain, d’un observateur accompli et soutenu du monde végétal, d’un expérimentateur botanique rigoureux, et de l’éminent théoricien de l’évolution végétale.

La botanique a joué un rôle crucial à chaque étape de la vie de Darwin. En tant qu’étudiant, il collecta des spécimens pour l’herbier de son professeur de botanique lors d’une sortie géologique au pays de Galles. En voyage à bord du HMS Beagle durant cinq ans, il rassembla des plantes, ainsi que des os fossilisés et des dépouilles d’oiseaux. Dans sa préparation à la rédaction de L’origine des espèces, la botanique prit une place essentielle dans le développement de sa théorie de l’évolution. Il finit par transformer son domicile et les paysages environnants en une station botanique et se complut fortement dans son jardin expérimental. Les six volumes botaniques de Darwin allaient reprendre la phytologie en de nombreux points, à présent uniquement abordables à la lumière de l’évolution. Ses études sur la fertilisation des orchidées, sur les plantes insectivores et sur les plantes grimpantes ainsi que d’autres actions de végétaux ont toutes fourni des exemples très clairs et précis sur la manière dont l’évolution est en mesure de résoudre les traditionnels mystères de l’histoire naturelle. Par son travail, Darwin a posé les fondations de la botanique moderne, encore solides à ce jour.

Resumen

En su teoría de la evolución, Charles Darwin planteó que todos los seres vivos “estaban relacionados” ya que provenían de un ancestro común. De hecho, la rama del árbol de la vida que más llamó la atención de Darwin, fue la del reino vegetal, siendo éste una continua fuente de asombro. Estudió las plantas con gran consistencia y profundidad en el transcurso de una larga carrera científica. Fue un gran colector, un paciente y concienzudo observador del mundo vegetal, un riguroso botánico experimental y el mejor teórico de la evolución de las plantas.

La botánica jugó un papel central en cada una de las fases de la vida de Darwin. Como estudiante, colectaba plantas para el herbario de su profesor de botánica mientras desarrollaba actividades de geología en Gales. En su travesía de cinco años a bordo del Beagle, colectó tanto plantas como huesos fósiles y pieles de aves. Preparándose para escribir El origen de las especies, la botánica fue un elemento crítico para el desarrollo de su teoría de la evolución. Se sabe que convirtió su casa y los terrenos aledaños en una estación botánica la que con un enorme gusto fue su jardín experimental. Los seis libros de botánica escritos por Darwin abarcan diversas áreas de la biología vegetal hoy en día solo entendibles bajo la luz de la evolución. Sus estudios en la fertilización de orquídeas, plantas insectívoras, trepadoras y otros movimientos de las plantas, constituyen un ejemplo bellamente articulado de cómo la evolución solucionó algunos de los misterios dentro de la historia natural. A través de su trabajo, Darwin sentó los fundamentos de la botánica moderna, los cuales a la fecha son vigentes.

Professor David Kohn
Darwin Digital Library of Evolution
AMNH Research Library
American Museum of Natural History
Central Park West at 79th Street New York NY 10024
USA
Email: dkohn@amnh.org 

 Professor David Kohn is Oxnam Professor of Science and Society, Emeritus at Drew University, USA. He is also a Research Associate at the American Museum of Natural History Library, a Senior Research Fellow of the Charles Darwin Trust and a Life Member of Clare Hall, Cambridge University.